1. Field of the Invention
This invention relates to supercontinuum generation, and more particularly to an IR supercontinuum source using low-loss heavy metal oxide glasses.
2. Description of the Related Art
Broadband light sources have a wide range of applications in optical systems and particularly fiber optic systems. These sources may be used to generate wavelength division multiplexed (WDM) signals or for optical spectroscopy of materials, fiber-optic sensing, optical coherence tomography (OCT), fiber optic gyroscopes, frequency metrology, optical component testing, dispersion characterization of specialty fibers, and optical interrogation of fiber bragg gratings. Conventional broadband light sources include superluminescent semiconductor diodes, rare-earth doped ASE sources, optical parametric amplifiers, quantum cascade laser, free electron lasers and supercontinuum sources. These sources are evaluated based on many factors including output spatial mode, optical bandwidth, spectral power density, flatness of the output power over the bandwidth, the temporal pulse width, the energy per pulse, the repetition rate, the time-averaged output power over the bandwidth, the efficiency of the source, and the cost, size, weight, and reliability,
In a supercontinuum source, an optical pump signal interacts non-linearly with a medium such as an optical fiber to produce new frequencies leading to significant spectral broadening of the original pump signal. The pump signal is typically injected into the medium in the form of optical pulses having pulse widths on the order of pico-seconds or femto-seconds and high peak power.
Supercontinuum generation is possible in step-index optical fiber or dispersion shifted fiber. However, to achieve maximum supercontinuum bandwidth and the lowest pump power threshold for supercontinuum generation, the pump wavelength must be near the zero dispersion wavelength (ZDW) of the fiber. The waveguide structure of the fiber may be configured to shift the ZDW of the fiber away from the zero material dispersion wavelength (ZMDW) of the bulk glass to the pump wavelength. Photonic crystal fiber (PCF) is widely used over conventional solid core/clad fiber in that the dispersion properties of PCF can be easily tailored by manipulating the microstructure.
Supercontinuum generation has been demonstrated in silica photonic crystal fiber in the visible (0.39-0.75 microns) and near infrared (NIR) (0.75-2 microns). In silica photonic crystal fiber the multiphonon edge of silica glass limits the transmission window in the IR. This, in turn, limits the extent of spectral broadening in the IR. Supercontinuum sources on the market today such as from Fianium Ltd, NKT Photonics, Leukos, provide a supercontinuum in the visible and NIR that spans from about 400 nm to about 2.2 microns.
Many applications exist for broadband sources beyond about 2 microns into the Mid IR (MIR) from 2-5 microns and possibly Long Wave IR (LWIR) above 5 microns. Mid-infrared sources are key enabling technology for various applications such as remote chemical sensing, defense communications and countermeasures, and bio-photonic diagnostics and therapeutics. Researchers have experimented with other glasses such as fluorides, oxyfluorides, chalcogenides and heavy metal oxides such as tellurium oxide to generate supercontinuum into the MIR range. The non-linearity of these glasses is typically much greater than that of silica based glasses. These researchers have demonstrated supercontinuum generation above 2 microns with these glasses. However, the losses exhibited by these glasses in the upper MIR wavelengths from about 3 to about 5 microns persist at levels that are too high for uniform and efficient supercontinuum generation. To date, no one has demonstrated a supercontinuum source that generates supercontinuum over the MIR band from about 2 to about 5 microns with sufficient flatness and time-averaged power over the band to be commercially viable.